restore preference for SelectorEventLoop on Windows

[minrk]

tornado should still work better with Selector, even though 6.1 isn't broken with proactor. Picking Selector avoids the extra thread, which runs a Selector anyway.

So the choice is really between:

• Proactor in main thread for coroutines, but ~all IO in a Selector in another thread
• one Selector loop for everything

cf jupyter/notebook#6052

[codecov-commenter]

Codecov Report

Merging #513 (928f9dd) into master (627898f) will decrease coverage by 0.04%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #513      +/-   ##
==========================================
- Coverage   77.06%   77.01%   -0.05%     
==========================================
  Files         107      107              
  Lines        9495     9511      +16     
  Branches     1027     1032       +5     
==========================================
+ Hits         7317     7325       +8     
- Misses       1808     1814       +6     
- Partials      370      372       +2     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	66.77% <11.11%> (-0.47%)	⬇️
jupyter_server/pytest_plugin.py	90.76% <33.33%> (-0.96%)	⬇️
jupyter_server/_version.py	87.50% <0.00%> (-12.50%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	80.79% <0.00%> (+0.72%)	⬆️

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[vidartf]

I think the test failures here are significant, especially since I would consider them unexpected.

[vidartf]

From running locally: The test that will fail is always the first test to run the using the clever fixtures.

[vidartf]

I added a commit that fixes the tests, but I'm not a 100% on why it fixes it (I have some vague hand-wavy idea, but not a solid understanding).